A magnetoresistive device which utilizes magnetoresistance effect by which a resistance is varied by magnetic field applied is used for a sensor for detecting magnetic field, a magnetic head or the like. Conventionally, the magnetoresistive device employs a magnetic alloy thin film which is typically made of permalloy. Though the magnetoresistive device utilizes the difference of resistance which occurs depending on the relative angle between current direction and magnetization direction, the magnetoresistance ratio is as small as about 3 to 4%. Therefore, in order to increase sensibility, a material of large magnetoresistance ratio is desired.
Recently, as a new magnetoresistive element, a superlattice film made of with alternately layering a ferromagnetic material such as Fe, Co and a non-magnetic material such as Cr, Cu in a few nanometers period is known. The magnetoresistance effect by the superlattice film is established when the magnetization arrangement of the ferromagnetic layers which are separated by the non-magnetic layer changes from antiferromagnetic to ferromagnetic according to the increasing of the magnetic field applied, thereby obtaining the magnetoresistance ratio more than 10% at room temperature. It is called a giant magnetoresistance effect to distinguish from the conventional magnetoresistance effect. As the superlattice film showing the giant magnetoresistance effect, the films made of Fe/Cr, permalloy/Cu/Co/Cu, Co/Cu and the like are known. Since these superlattice films have a larger magnetoresistance ratio than the permalloy thin film which is conventionally known, they are expected to significantly improve the properties when they are applied to the magnetic sensor or head. However, in the superlattice films reported until now, they show a high value of the saturation magnetic field Hs of a few kOe to about 10 kOe which means saturation of magnetoresistance in comparison with that of permalloy of a few Oe. Therefore, it is difficult for them to apply to a magnetic sensor or head which requires sensibility of magnetic field.
On the other hand, inducing a uniaxial in-plane magnetic anisotropy to a superlattice film is suggested as a method for decreasing the saturation magnetic field of the superlattice film while maintaining the high magnetoresistance ratio thereof. For instance, Japanese Patent Application Laid-Open No. 4-212402 discloses a method for inducing a uniaxial in-plane magnetic anisotropy to a superlattice film where the magnetic field of about 100 Oe is applied to the film surface by a permanent magnet when Fe/Cr superlattice film is prepared. Furthermore, W. Folkerts and F. Hakkens, "Microstructure Induced Magnetic Anisotropy in Fe/Cr(110) Superlattices" J. Appl. Phys. vol.73, pp.3922-3924 (1993) describes a method for inducing a uniaxial in-plane -anisotropy to a superlattice film by the structural magnetic anisotropy which is caused by the microstructure provided with a Fe/Cr (110) superlattice film. As compared with the former method, the latter method in which the uniaxial magnetic anisotropy is induced with utilizing the structural magnetic anisotropy is more effective to decrease the saturation magnetic field. However, the level of the saturation magnetic field is still so high as 500 Oe at room temperature.
As described above, the Fe/Cr (110) superlattice which has the microstructure has a disadvantage that the level of the saturation magnetic field is too high whereas it has the large magnetoresistance ratio. Thus, when it is employed for a device such as a magnetic sensor or head in which a low magnetic field is detected, the device can not obtain enough sensibility.